In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent properties with respect to heat resistance, impact resistance and transparency. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound and phosgene has been commercially practiced, wherein 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as “bisphenol A”) can be mentioned as a representative example of the aromatic dihydroxy compound.
However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous, that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride, which are by-produced, and methylene chloride which is used as a solvent in a large quantity, and that difficulties are encountered in separating and removing impurities (such as sodium chloride) and residual methylene chloride, which adversely affect properties of a produced polymer.
For solving such problems, there have been proposed many methods in which, instead of phosgene, a dialkyl carbonate or a diaryl carbonate is used to produce an aromatic polycarbonate. Examples of methods in which a dialkyl carbonate is used to produce an aromatic polycarbonate include (i) a method in which a dialkyl carbonate and an aromatic dihydroxy compound are subjected to transesterification to thereby obtain an aromatic polycarbonate (see patent documents 1, 2 and 3), and (ii) a method in which a dialkyl carbonate and an aliphatic ester of an aromatic dihydroxy compound are subjected to transesterification to thereby obtain an aromatic polycarbonate (see patent document 4). In the case of a method using a dialkyl carbonate, during the reaction, a fraction containing an aliphatic monohydroxy compound or a derivative thereof and a dialkyl carbonate or the like is obtained from the reaction system and withdrawn as a column top distillate from a distillation column. Since such components of the column top distillate are usually liquid at room temperature, it is easy to handle the components, as compared to the case of the methods using a diaryl carbonate. However, the method (i) above has a defect in that the reaction rate is low and, hence, it is difficult to obtain a high molecular weight polymer, and the method (ii) above has problems not only in that an unstable and toxic substance, such as ketene, is byproduced in the production of the raw material, but also in that the procedure of the method (ii) is complicated, thus rendering the method (ii) unsatisfactory for use as a commercial process. Further, the methods using a dialkyl carbonate pose a quality problem in that the aromatic polycarbonate produced has a terminal alkyl carbonate group, and such aromatic polycarbonate has poor heat stability.
A method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate by transesterification has conventionally been known. For example, an aromatic polycarbonate can be produced by performing a molten state polymerization between bisphenol A and diphenyl carbonate. In this method, for achieving a high degree of polymerization with respect to a polycarbonate produced, it is necessary to remove by distillation an aromatic monohydroxy compound (such as phenol or the like) from a molten polycarbonate having high viscosity. This method has defects, for example, in that (1) since the polymerization is performed at a high temperature, branching and crosslinking are likely to occur due to side reactions, thus rendering it difficult to obtain a high quality polymer, and that (2) discoloration cannot be prevented (see non-patent document 1).
For overcoming these defects, many proposals on catalysts, stabilizers, polymerization methods and the like have been made. For example, in patent document 5, the present inventors disclosed a method in which a molten mixture of an aromatic dihydroxy compound and a diaryl carbonate or a prepolymer obtained by reacting an aromatic dihydroxy compound with a diaryl carbonate is subjected to free-fall polymerization in which the prepolymer or molten mixture is allowed to fall freely, thereby effecting a polymerization during the free fall. By this method, a high quality polycarbonate having no discoloration can be produced.
The properties of a resin, such as color, are expressed by a wide variety of indexes. However, the basic properties of a resin (such as viscosity) are likely to depend on the molecular weight of the resin. Therefore, it is an important task to develop a technology which is useful for efficiently producing a resin having a desired molecular weight. In addition, a resin has also properties (such as moldability and heat resistance) which are greatly influenced not only by the molecular weight but also by, e.g., the ratio of terminal hydroxyl groups. For this reason, it is also an important task to establish a method which is useful for stably controlling the terminal hydroxyl group ratio of a resin.
In the case where an aromatic dihydroxy compound and a diaryl carbonate are subjected to molten state polymerization, for the purpose of realizing a resin having such desired properties as a stable molecular weight, a stable ratio of terminal hydroxyl groups, and freedom of discoloration, it is especially important to provide a stable molar ratio of an aromatic dihydroxy compound and a diaryl carbonate. Therefore, it has conventionally been attempted to develop a technology to stabilize the molar ratio of an aromatic dihydroxy compound and a diaryl carbonate (see patent documents 6, 7 and 8). However, no satisfactory technology has been obtained. Therefore, it has been desired to further improve the technology to stabilize the molar ratio of an aromatic dihydroxy compound and a diaryl carbonate.
[patent document 1] Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-2334
[patent document 2] Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169444
[patent document 3] Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169445
[patent document 4] Unexamined Japanese Patent Application Laid-Open Specification No. Sho 59-210938
[non-patent document 1] “Purasuchikku Zairyo Koza [5]: Porikaaboneito Jushi (Lectures on Plastic Materials [5]: Polycarbonate Resin)”, written by Mikio MATSUKANE et al., p. 62 to 67, published in 1969 by The Nikkan Kogyo Shimbun Ltd, Japan
[patent document 5] International Publication No. W095/03351
[patent document 6] Unexamined Japanese Patent Application Laid-Open Specification No. 2003-183379
[patent document 7] Unexamined Japanese Patent Application Laid-Open Specification No. 2003-201343
[patent document 8] Unexamined Japanese Patent Application Laid-Open Specification No. 2003-192782